Semiconductor wafers are subjected to a variety of processing steps in the course of the manufacture of semiconductor devices. The processing steps are usually carried out in sealed vacuum chambers of wafer processing machines. Most of the processes performed on the wafers require the monitoring and control of the temperature of the wafers during processing, and certain of these processes, such as degassing and annealing processes, have the heat treating of the wafers as their essential process step. In several of such processes, particularly the essentially thermal treatment processes, a plurality of wafers may be stacked in a rack within a chamber of the processing machine and simultaneously processed as a batch.
A variety of temperature sensing techniques are employed in the various semiconductor wafer treating processes to monitor the temperature of the wafers and often also to control the wafer heating or cooling elements. Thermocouple devices, for example, are frequently employed, particularly when a wafer is being treated while held in thermal contact with a temperature controlled wafer support. In such cases, the thermocouple is often maintained in contact with the support, and thus only indirectly measures the temperature of the wafer on the support. In certain other situations, thermocouple devices, are brought in direct contact with the wafer. Such positioning of the thermocouples may expose the sensors to heat directly from the wafer heating source, such as where radiant energy is used to heat the wafer, or may, by direct contact with the wafer, contribute to undesirable wafer contamination.
Techniques have also been proposed for deriving wafer temperature indirectly by measuring the thermal expansion of the wafer. Such techniques present a disadvantage in that such measurements yield a reading proportional to temperature difference. Accordingly, initial wafer temperature must be known and a wafer dimension must first be measured at the known initial temperature before the monitored temperature can be derived. Furthermore, such techniques can be effective to read the temperature of a single wafer, but these techniques are difficult to apply where a plurality of wafers, particularly closely spaced wafers, are processed and the temperature of the wafer batch must be read.
In many wafer processing machines, pyrometers are employed to measure the temperature of wafers being processed within. These pyrometers measure the emissive power of heated objects such as the wafers. This emissive power, however, varies with the emissivity of the object, which, for some materials, varies with temperature. The emissivity particularly varies with the materials of which the object is made and of the coatings which have been applied to the object. In semiconductor wafer processing, there are many kinds of coatings that may be found on the wafers. These coatings vary with the processes used on the wafer. Accordingly, for a pyrometer to be used accurately to measure the temperature of such a coated wafer, an initial measurement to determine the emissivity of the object is frequently required.
As a result of the problems with pyrometers, a number of schemes to measure wafer temperature in semiconductor wafer processes have been devised that either measure the emissivity of the object or apply some sort of a correction to the pyrometer output. Often a pyrometric temperature is measured of an object of known emissivity mounted in the chamber and known to be at the same temperature as the wafer being measured. Often also, a measurement is made with a reference pyrometric sensor to generate data that is then used to correct the temperature reading from a primary parametric sensor to account for the emissivity of the wafer being measured.
Measurement of wafer temperature from the backside of a wafer may reduce the effect of the emissivity changes due to the coatings on the front side of the wafer, but an initial problem is encountered in that the uniformity of the backside of the wafer is not precisely controlled, and may vary from wafer to wafer.
Accordingly, there remains a problem of accurately measuring the temperature of semiconductor wafers or similar thin flat articles during processing in semiconductor wafer processing machines. Particularly, there is a need for measuring, without contact with the wafer, the temperature of wafers, particularly where they are being thermally processed in batches and may be closely spaced in a stack within the processing chamber.